 Chapter C apte 2 ZENER DIODE

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

1

Analysis of networks employing Zener diodes is quite similar to that applied to the analysis off semiconductor i d t diodes di d iin previous i sections. ti Here, first the state of the diode must be determined followed by a substitution of the appropriate pp p model. Then, the other unknown quantities of the network will be determined.

Equivalent circuit

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

2

(i) Vi and R Fixed

Basic zener regulator Analysis procedure Step (1) Determine the state of the zener diode by removing it from the network and calculating the voltage across the resulting open circuit.

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

3

Step (2) Substitute the appropriate equivalent circuit and solve for the desired unknowns. Ex: In case of “on” state.

Voltages across the parallel elements must be the same.

When the system is turned on on, the Zener diode will turn “on” on as soon as the voltage across The Zener diode is Vz volts. It will then “lock in” at this level and never reach the higher Level of V volts. Zener diodes are most frequently used in regulator networks or as a reference voltage. Regulator is designed to maintain a fixed voltage across the load RL. For values of applied voltage greater than required to turn the Zener diode “on”, the voltage across the load will be maintained at Vz volts. volts

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

4

Example (1): (a) determine VL, VR, Iz and Pz. (b) change RL to 3 kOhm.

Solution (a) Step (1) Removing the diode

V < Vz The diode is “off”

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

5

Step (2) Substitute open-circuit equivalent.

Solution (b) ( ) Step (1) Removing the diode

V > Vz , the diode is “on” Step (2) Substitute the equivalent circuit.

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

6

(ii) Fixed Vi, Variable RL Too small a load resistance RL will result in a voltage g VL across the load resistor less than Vz and the Zener device will be in the “off” state. To determine the minimum load resistance that will turn the Zener diode on, on simply calculate the value of RL that will result in a load voltage VL = Vz.

Any load resistance value greater than the RL obtained above will ensure that the Zener diode is in the “on” state and the diode can be replaced by its Vz source equivalent. equivalent Then, we obtain The Zener current

Once the diode is in the “on” state, the voltage g across R remains fixed at

Resulting in a minimum Iz when IL is a maximum and a maximum Iz when IL is a minimum value since IR is constant. constant

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

7

Example (2) (a) Determine the range of RL and IL that will result in VRL being maintained at 10V. ( ) Determine the maximum wattage rating off the diode. (b)

Solution (a): First determine the value of RL that will turn the Zener diode on First, on.

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

8

Th range off RL and The d IL that th t will ill resultlt iin VRL being b i maintained i t i d att 10V 10V.

Solution (b):

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

9

(iii) Fixed RL, Variable Vi For fixed F fi d value l off RL, the th voltage lt Vi mustt be b sufficiently ffi i tl large to turn the Zener diode on. The minimum turn-on voltage Vi = Vimin is determined by

The maximum value of Vi is limited by the maximum Zener current IZM. Since IZM = IR â&#x20AC;&#x201C; IL,

Since IL is fixed at Vz/ RL and IZM is the maximum value of Iz, the maximum Vi is defined by

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

10

Example (3): Determine the range of values of Vi that will maintain the Zener diode in the “on” on state. state

Solution:

Chapter 2 ZENER DIODE

http://ebspac.blogspot.com/

11

Advertisement